Secure documents such as currency, passports, or identity cards are increasingly counterfeited around the world. This situation is a very critical issue for governments and society in general. For example, criminal organizations may use fake passports or identity cards for trafficking of human beings. As reprographic technologies become more and more sophisticated, it becomes even more difficult to make a clear distinction between a fake document and the original. Document security has therefore a considerable impact on the economy of countries and also on victims of illicit traffic involving counterfeit documents.
Passports and identity cards are in general secure documents which contain a large number of protections, such as holograms, bar codes, encrypted data, specific papers or substrates, etc. Some protections are visible to the unaided eye (“overt” features), other protections are invisible (“covert” features) and their detection requires specific equipment.
Paper related documents which have valuable interest such as banknotes, fiduciary documents or even recently diplomas, such as diplomas from high schools and colleges and universities, are highly subject to counterfeit. In university or college or high school, for example, the level of security of diplomas made with paper is not enough to prevent illicit reproduction. Very often in such documents there is found as a control element the stamp of the college or university or high school, and eventually a hologram as a security feature. However, holograms are more susceptible to easy reproduction as printing techniques become more and more sophisticated. This permits counterfeiters to reproduce such documents of value, including the college or university or high school stamp.
In order to prevent such diversion there is a need to find alternative solutions to protect such documents, especially documents made with paper. Security inks comprising luminescent compounds are known to prevent such diversion and very often inkjet compositions are used to protect documents of value.
In US 2007/0225402 A1, which is incorporated by reference herein in its entirety, the use of an ultraviolet luminescent ink is disclosed, which is printed in form of indicia onto the document. The ultraviolet luminescent ink is invisible under natural light, such that the indicia can be only revealed under irradiation with UV light. This ultraviolet luminescent ink is useful for applying codes onto security documents such as passports or banknotes. In the cited document, printing processes, including silk screen, gravure, letterpress and offset printing are used to apply the invisible ultraviolet fluorescent inks.
Luminescent compounds in pigment form have been widely used in inks and other preparations (see U.S. Pat. No. 6,565,770, WO 2008/033059 A2, WO 2008/092522 A1). Examples of luminescent pigments can be found in certain classes of inorganic compounds, such as the sulphides, oxysulphides, phosphates, vanadates, garnets, spinels, etc. of nonluminescent cations, doped with at least one luminescent cation chosen from the transition-metal or the rare-earth ions.
Another class of compound useful to produce luminescence in ink is formed by certain rare-earth metal complexes such as described in WO 2009/005733 A1 and its family member US 2009/0000509 A1 or in U.S. Pat. No. 7,108,742, which are incorporated by reference herein in their entireties.
A particular process for imprinting secure documents with luminescent compounds, in particular luminescent rare-earth metal complexes, is inkjet printing, and more particularly thermal inkjet printing. Thermal inkjet printers use print cartridges having a series of tiny electrically heated chambers, constructed by photolithography. To produce an image, the printer sends a pulse of electric current through heating elements disposed in the back of each chamber, causing a steam explosion in the chamber, so as to form a bubble, which propels a droplet of ink through an orifice of the chamber onto the paper in front of it (hence the tradename Bubblejet® for certain inkjet printers). The ink's surface tension, as well as the condensation and thus contraction of the vapor bubble, pulls a further charge of ink into the chamber through a narrow channel attached to an ink reservoir.
The ink used is aqueous (i.e., a water-based ink comprising pigments or dyes), and the print head is generally cheaper to produce than the equipment required for other inkjet technologies. However, its lifetime is short, and it is generally exchanged together with the empty ink cartridge.
A major problem encountered with inkjet printers is ink drying in the printhead's nozzles, causing the pigments and/or dyes to form a solid deposit that plugs the microscopic ink orifices. Most printers prevent this drying by automatically covering the printhead nozzles with a rubber cap when the printer is not in use. Abrupt power loss, or unplugging the printer before it has capping of the printhead can, however, cause the printhead to dry out. Further, even when capped—this seal being not perfect—, over a period of several weeks, the ink in the nozzles can dry out and plug them. Once ink begins to dry out in the nozzles, the drop volume is affected, the drop trajectory can change, or the nozzle can completely fail to jet any ink.
In the case of luminescent inkjet inks comprising rare earth metal complexes, the stability of the complex in water is critical to avoid nozzle obstruction. In order to prevent premature drying, adding water or solvent, to sufficiently dilute the ink, is an obvious solution. However, dilution with water or solvent reduces the intensity of luminescence (and thus the ease of detection) of the security document printed with such ink.
Another problem also very often encountered when using ink which comprises luminescent pigments when used, for example, in a water based ink composition with inkjet printer (very often used for printing security document or packaging of value made with paper), is their stability as well as degradation of the ink inside the nozzle of the printer by known mechanisms such as Kogation or precipitation.
Although rare earth metal complexes would represent a very useful way of imparting luminescence to inkjet inks, the problem of ink drying in the nozzles makes it often impossible to use the inkjet cartridges in their entirety, and causes thus increased ink cartridge consumption cost. This has not only an ecological and security impact, due to the “recycling” problem caused by such “used” cartridge, but also a non-negligible impact on the cost of printing.
US 2010/0307376 A1, which is incorporated by reference herein in its entirety, is directed to a lanthanide chelates and secure documents containing lanthanide chelates, and provides many advantages and overcomes many of the disadvantages of the prior art, especially disadvantages resulting from stability, premature drying and Kogation.
Despite the considerable interest of such luminescent inks, the paper which must be protected remains a problem, because not all luminescent inks will adhere to the surface of the paper. Additionally, more problematic is the fact that the ink used when printed onto paper type documents or items does not confer to the paper a high level of security. The security element present in such ink when printed to the paper is not fully revealed or its signal remains relatively weak which does not allow a full recognition of the genuine paper type document or items.
There is therefore still a need to have a specific and selected water based composition which allows to provide a high level of security when printed on a paper substrate, which allows allow an efficient level of recognition of the security element with a specific luminescent profile hard to reproduce comprised in water based ink composition and which also avoid the drawbacks of instability, kogation and precipitation inside the nozzle of the ink jet printer.
Thus, there is still a crucial need to solve the above mentioned problems in order to promote the efficient use of luminescent ink-jet inks based on rare earth metal complexes, and therefore to obtain correctly printed and protected security documents during the whole life of the ink cartridge.